1. Technical Field
This invention relates to the field of audio reception and transmission systems generally and, more particularly, to the field of voice switched gain systems for loudspeaking telephone or hearing aid applications involving electret microphones.
2. Description of the Prior Art
It is recognized in the art of loudspeaking telephones and voice conferencing arrangements that background noise is undesirable and can present a situation where intelligible audio control may be easily lost to the noise source. The noise itself should be cancelled or eliminated. On the other hand, switching of control from near end to far end or from audio source to audio source should be smooth and free of chopping as, for example, when whole first syllables of a word spoken may be lost due to a slow switching process. As a result of the development of the electret microphone, a smooth, linear, highly reliable gain characteristic is achievable in such applications, but telephone engineers have been generally reluctant to implement electret devices in such applications because the gain characteristic was likewise applied to noise as to desirable voice.
Matthews, in U.S. Pat. No. 3,730,995, discloses a Voice Controlled Microphone Control System including a speech detection circuit and relay arrangement for preempting control of a conference. When speech above a particular level is detected, a relay is operated and a signal is generated which precludes transmission by other microphones of the system so long as the speech level exceeds the particular reference level. No solution is suggested for dealing with background noise problems, however.
The Loudspeaking Telephone disclosed by Breeden in U.S. Pat. No. 3,751,602 tackles timing problems of gain switching and introduces two modes of operation--a transmit or a receive mode where the mode is changed when sufficient audio level is detected from the non-controlling source. Little concern is given to background noise which still can take control of the Breeden telephone. Furthermore, with changes in background noise level, the party who desires control always wonders if they really have control or whether their transmission has been interrupted by noise at the far end.
In Reid, U.S. Pat. No. 3,610,835, a loudspeaking telephone is disclosed which includes a variable impedance device for increasing loss when a translated received noise is greater than a translated received speech signal. This means of dealing with a noise source at the near end does not take into consideration the characteristic gain of electret microphone devices and encourages the near end user to shout to preempt control form the noise source. Also in Reid, there is always a default direction of transmission or controlling end, namely the near or receive end, but usually the near end, which is only switched from when control is taken by the far end. In this arrangement, the far end sounds dead during idle periods to the near end.